Recombinant gene therapy techniques have been developed and evaluated in preclinical and clinical tests over the last fifteen years. Gene therapy techniques are particularly promising for treating diseases or deficiencies that are caused by a deficiency or abnormality in the expression of a particular gene in specific cell or tissue types because a gene of interest can be placed under the control of a cell-specific or tissue-specific promoter to achieve location-specific expression of that gene. In particular, there has been significant interest in developing gene therapy techniques to treat liver-specific diseases or deficiencies.
Multiple recombinant gene transfer vectors based on different types of viruses have been developed and tested in clinical trials in recent years. Gene transfer vectors based on adeno-associated virus (AAV) have become favored vectors because of characteristics such as an ability to (i) transduce different types of dividing and non-dividing cells of different tissues, and (ii) establish stable, long-term transgene expression. While vectors based on other viruses, such as adenoviruses and retroviruses, also posses such desirable characteristics, the use of other vectors has been associated with toxicity of some human diseases. These side effects have not been detected with gene transfer vectors based on AAV. Additionally, the technology to produce and purify AAV-based vectors without undue effort has been developed.
AAV-based vectors have provided encouraging results for providing a method of liver-specific gene expression. For example, clinical trials showed that a recombinant AAV vector containing a factor IX (FIX) sequence was able to produce a therapeutic circulating level of FIX protein from hepatocytes in hemophilia B patients. A deficiency of FIX, which is a coagulation factor normally produced by the liver, is the cause of hemophilia B. Hemophilia B is an ideal disease target for gene therapy because it is due to a single gene mutation and because a small increase in the circulating levels of FIX is sufficient to measurably reduce the symptoms of hemophilia B. However, it was observed in the clinical trials described above that the circulating level of FIX protein dropped below therapeutic levels after several weeks (Wang, L. et al., Curr. Gene Ther. 5:349-360 (2005); High, K. A., Semin. Thromb. Hemost., 30:257-267 (2004)). It is believed that the high doses of the recombinant AAV vector that were used triggered an immune response against the transduced hepatocytes.
Thus, there remains a need for providing liver-specific expression of a gene. In particular, there is a need for liver-specific expression constructs that provide robust expression levels from viral vectors that do not trigger an inactivating immune response.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.